I used to use Outlook as my main client, several years ago.  I did not have any big complaints.  It was functional and reasonable.  It did what I needed.

I switched to web mail primarily for the convenience of not having to lug my computer around while traveling.  I am a big fan of traveling light, so being able to cut out the lap top and its accessories was a big improvement.

I switched to GMail when it came out just for experimentation.  Gradually I started forwarding my email from other accounts to GMail.  I’ve been very happy with GMail, and I’m only getting happier as they add features.  I don’t know if I’ll use the new Multi-Pane Display for example, which seems like a feature out of the Outlook play book, but it’s nice to see ongoing innovation in the product.

Now it’s at the point where I just can’t fathom using Outlook.  I see people at work using it, and they certainly tolerate what now appears to me as its glacial speed.  But I can’t do it.  It’s so slow as to be infuriating.

I love the keyboard shortcuts.  I’m so much more productive sorting, filtering, archiving, searching, and doing all the classic message management functions with just the keyboard.  Plus the shortcuts are customizable easily.  Plus they’re consistent with Google Reader, which I also use.  It’s just a great productivity / efficiency boost.  I see people left- and right-clicking multiple times per message read / manipulated, and it just seems kind of strange.

I’d switch to another email provider if it were as fast as keyboard-friendly as GMail.  It’s not a particular Google allegiance.  But it would certainly have to be a good mail client.